Big data is a term for data sets that are so large or complex that traditional data processing applications are inadequate. Challenges include analysis, capture, data curation, search, sharing, storage, transfer, visualization, querying, updating and information privacy.

But before I explain what the different SEO metrics in Ahrefs mean (and how we calculate them), I’d like to brag for a bit about the monstrous infrastructure that’s running behind the scenes.

Ahrefs Data Index

Here are a few core numbers, representing the size of Ahrefs’ index (you can always find them on our homepage and on the Our Data page):

Creating a robot that will crawl the web and store web pages on your hard drive might sound like an easy thing to do.

But that’s until you try to achieve the crawl speed of 200 million pages per hour. Or try to store all this data in such a way that your customers can make a call to the database and have all their graphs and reports built in seconds.

That’s why the majority of our backend infrastructure was built in-house. All existing solutions simply couldn’t keep up with the volume of data we operate or, if they could, were too expensive. As of today, we’re running a custom big data database with ~85 trillion rows.

URL Rating (UR)

“URL Rating” measures the strength of a target URL’s backlink profile and the likelihood that the URL will rank high in Google.

UR is measured on a logarithmic scale from 1 to 100, with the latter being the strongest.

Sidenote.

“Logarithmic” means that it’s much easier to grow your page from UR 20 to UR 30 than from UR 70 to UR 80.

We often see people explain Ahrefs’ URL Rating as a replacement for Google’s PageRank metric, but that’s not entirely accurate. UR and PageRank aren’t the same things.

We indeed started out with a PageRank-like formula, but then “UR” underwent quite a few iterations with a goal of creating a metric that would have the highest possible correlation with Google rankings.

And, as you can tell from the graph below, the URL Rating correlates with Google rankings better than any of our “unprocessed” backlink metrics:

Moz has a similar metric to our URL Rating called Page Authority, which predicts the likelihood of a page ranking high in Google. And, according to their own research study, it correlates with Google ranking slightly worse than our metric.

Domain Rating (DR)

“Domain Rating” shows the strength of a given website’s overall backlink profile.

DR is measured on a logarithmic scale from 1 to 100, with the latter being the strongest.

Domain Rating correlates with Google rankings pretty well, but not as well as the URL Rating—which suggests that you might be able to outrank high-authority sites if you build more backlinks to your page.

DR is a great metric for picking websites to build links from. As a general rule, you need to aim to get backlinks from high-DR websites because they carry more “weight.”

Here are a few popular questions our customers ask about DR:

Q1:“I didn’t lose any of my backlinks. Why did my DR drop?”

A1:“That’s because other sites have gained a lot of backlinks. Think of it this way: when a DR-100 website gets more backlinks, we can’t make it DR-101. So instead we push all the other websites down by 1. That’s a very raw explanation of why you might see a drop in your DR while no backlinks were lost.”

Q2:“My competitor doesn’t have links from high-DR sites, so why is his DR higher than mine?”

A2:“If a high-DR website links to thousands of other sites, we value their links less. But if a low-DR website only links to a few sites, we consider these links quite powerful. Your Domain Rating doesn’t solely rely on the number of high-DR sites that link to you. It also takes into account how many other websites these high-DR sites link to.”

Ahrefs Rank (AR)

If you take all the websites in the world and order them by the size and quality of their backlink profile (basically by their DR), you’ll get the Ahrefs Rank.

So Ahrefs Rank #1 belongs to the website with the best backlink profile, #2 is just a bit worse, #3 is a little worse than that, etc.

You can see the full list of domains sorted by their Ahrefs Rank here.

Here’s one other way to understand it.

Alexa Rank shows you how much traffic a website has relative to other websites in the world. And Ahrefs Rank shows you how good the website’s backlink profile is relative to all other sites in the world.

Calculating the overall relevancy of a page might be a good addition to KD, but we haven’t tackled it just yet.”

Q2:“The #1 result is very strong! And your KD is way off!”

A2:“Ahrefs’ Keyword Difficulty score only shows you how hard it would be to rank in the top10, not #1. When you’re in the top10, a lot of other ranking factors kick in, so it’s almost impossible to accurately calculate your chances of ranking #1.”

Q3:“How accurate is your KD score compared to other tools?”

A3:“It’s impossible to objectively compare KD scores from different tools because there’s no “base KD value” to compare against. Each SEO professional uses his “gut feeling” as a base value, which obviously differs from person to person.

And I plan to write a dedicated article about the issue with Google’s search volume and how we solve it. So if you don’t want to miss it, please subscribe to the Ahrefs Blog.

Organic Search Traffic

For any website you put into Site Explorer, we’ll show you the number of keywords it ranks for and the estimated amount of organic search traffic it gets:

And if you switch to the “Organic search” tab of the “Overview” report, you’ll see this beautiful graph:

So how accurate are these numbers and how do we calculate them?

Organic keywords

The database of keywords in Site Explorer is approaching 200 millions for the United States alone.

But, obviously, even a database that big can’t cover all possible search queries that people might put into Google. That’s why we tend to underestimate the total number of keywords a website is ranking for.

Organic traffic

This is our estimation of how much traffic a target website or URL gets from organic search. We calculate it based on the keywords we see that website or URL ranking for.

And because we don’t have all possible keywords (see above), we tend to drastically underestimate the search traffic of a given website.

For example, for the Ahrefs Blog, we underestimate organic search traffic by 5x:

But as soon as you try to compare two websites from the same niche, you get a very accurate picture.

Ahrefs shows that our company blog gets around 10x more search traffic than my personal blog:

Which is exactly the case, if you look at the numbers from Google Analytics:

In other words, the organic traffic numbers you see in Ahrefs should not be treated as exact values but, rather, should be used for comparing websites with each other and seeing trends in your search traffic over time.

Back To You

So these are the metrics we get asked about the most.

I hope that this article helps you better understand Ahrefs’ reports and make them more actionable.

If you have any other questions about the numbers you see in Ahrefs, feel free to ask them in the comments and I’ll be happy to help you.